The purpose of the Nelfinavir versus Ritonavir Protocol is to determine whether a strategy of adding nelfinavir (NFV) to background antiretroviral nucleoside therapy is equivalent, in terms of delaying disease progression events or death, to the strategy of adding ritonavir (RTV) to background antiretroviral nucleoside therapy in patients with CD4+ cell counts < 100/mm3. The protocol document provides the background and rationale, the study design, the eligibility requirements, the sample size and the assumptions used to arrive at the sample size, the methodology for implementation of the study, and the plan for analysis of the data. This protocol is being carried out with the support of the NIAID and Agouron Pharmaceuticals, Inc. This study will be an open-label trial in which eligible patients will be randomized either to NFV plus background AR nucleoside therapy or to RTV plus background AR nucleoside therapy. Patients will be allowed to cross-over to the alternate protease inhibitor if they develop a disease progression event or a significant intolerance to the assigned agent.